Biomedical electrodes are used in a variety of applications and are configured to operate according to the size, type, and direction of current flowing into or out of a body of a patient.
Dispersive electrodes are used in electrosurgery. In modem surgical practice, there are many times when electrosurgery is more preferable than the use of the traditional scalpel. In electrosurgery, cutting is performed by an intense electrical current passing through a cutting electrode. The surgeon directs this current to exactly where cutting is required by wielding the cutting electrode, which because of its cylindrical shape and the way it is held in the hand is commonly called an "electrosurgical pencil". By activating controls which change the characteristics of the electrical current being sent to the pencil by an electrosurgical generator, the surgeon can use the pencil either to cut or to coagulate areas of bleeding. This makes electrosurgery particularly convenient when surgery requiring extra control of blood loss is being performed. Because of concerns to minimize the transmissions of blood-borne illnesses between health care patients and health care providers, in both directions, electrosurgery is becoming increasingly important.
In electrosurgery, as in all situations where electrical current is flowing, a complete circuit must be provided to and from the current source. In this case, the current that enters the body at the pencil must leave it in another place and return to the generator. It will readily be appreciated that when current enough to deliberately cut is brought to the body of a patient in one place, great care must be taken that unintentional damage is not also done to the patient at the location where that current is leaving the body. The task of collecting the return current safely is performed by a dispersive electrode.
A dispersive electrode performs this task by providing a large surface area through which the current can pass; the same current which was at cutting intensity when focused at the small surface area at the tip of the pencil is relatively harmless, with the goal of being painless to the patient, when spread out over the large surface area of the dispersive electrode.
Unfortunately, any geometry of the large surface area has an edge and perhaps distinct comers or junctions where "edge effects", caused by increased current density at those locations, can have a maximum temperature rise during usage by the patient making such dispersive electrode or cardiac stimulating electrode most uncomfortable to the patient.
The same difficulties concerning edge effect also are present in cardiac stimulating electrodes, such as those used for defibrillation, external pacing, or cardioversion. For a patient already in some discomfort or ill health, pain sensed by the very medical device intended to treat the patient is disconcerting at best.
Safety systems for the electrosurgical patient plates use "Contact Quality Monitor" ("CQM") circuits. All CQM systems currently in use are based on a single design, involving the use of a split patient plate. The reason that the plate must be split is to create two separate conductors that are not electrically joined, unless the electrode is placed on the skin of a patient. If it is indeed properly placed, then a small current can be passed by the generator down one wire of the cable connecting the generator to one of the conductors on the split plate. From there, the current then passes into the flesh of the patient, crosses over to the other conductor of the split plate, and then back through the other wire of the connecting cable to the generator. By analyzing this current, the generator is able to measure an impedance for the combined circuit of the cable, both halves of the split plate, and the patient.
This impedance must be within a certain pre-determined range which assures that the plate has been placed on the patient and that it is in full or nearly full contact with the skin of the patient.
One other point should be made about these CQM systems, and the current that is used by them to determine when the plate is in good contact with the patient: The CQM current should not be confused with the surgical current, which does all the cutting and coagulating. The CQM current is a lower frequency (typically about 39-350 KHz) than the surgical current frequency (typically about 500-1000 KHz), and is hundreds of times smaller in both voltage and amperage than the surgical current (about 1 mV and 1 mA vs. about 500 V and 2 A, respectively).
It is also important to note that the CQM system is only active when the surgical current is not flowing, since the surgical current is so powerful that it would burn out the CQM circuit if the CQM circuit were active while the surgical current was flowing.
The CQM system, first introduced in 1984, has become the industrial safety standard for electrosurgery. Further disclosure of the CQM system is found in U.S. Pat. Nos. 4,200,104 (Harris); 4,231,372 (Newton); 4,416,277 (Newton et al.); 4,416,276 (Newton et al.); and 4,848,335 (Manes).
In addition to the CQM system, additional systems have been developed. One of them is the "NESSY" system sold by Erbe, Inc. of Tubingen, Germany. The system has two separate circuits, with the first circuit being the standard CQM type described above. The second circuit, however, is unique to the Erbe generator and actually measures the surgical current that is flowing through both halves of the split plate. The amperage flowing in the two halves of the plate is compared, and if there is too great a difference between the current levels the generator will alarm and shut down.
Another attempt to provide protection for an electrosurgery patient is disclosed in U.S. Pat. No. 5,080,099 (Way et al.). In these patents are disclosed a triple plate electrode in order to provide a measure of "peel back" of the patient plate from the patient. But these electrodes disclosed in the Way et al. patent were quite complex to manufacture and use.